Recently, impedance measuring circuits have been known where input signals such as AC signals are input to a target such as a living body and impedance in the target is measured based on a magnitude of the input signal attenuated after passing the target. Also, by detecting a change in the magnitude of the input signal after passing the target, a change in the impedance in the target can also be measured. When a living body is the target, it is desirable to measure a change of, for example, 1Ω or less in the impedance and thus an amplitude of the input signal is also limited to be relatively small. Therefore, it is desirable to measure a very small change in the amplitude.
In such an impedance measuring circuit, it is desired to reduce power consumption.